The invention relates to tilting cab-over-engine type truck cabs, and more particularly to an improved rear mount and latching system for a tilt cab.
In the design of motor vehicles, passenger cars generally are provided with frames which are very rigid, while most trucks usually have limber frames which can give to some extent with the irregularity of the surface over which they are driven. Cab-over-engine, tilting cab trucks almost always have a limber chassis, and this can present special problems. The cab itself, which is essentially a thin sheet metal box, is relatively rigid. Twisting of the frame, as when the truck is driven angularly onto an incline, or pulled off the side of the road or in any similar situation, affects the cab and would damage the cab if the cab were fixedly secured to the frame at all four points. The tilt cab of course pivots about a front axis usually located just above the front bumper. At the rear, where latching is necessary, manufacturers have utilized some type of spring-loaded device to provide for a degree of up/down travel at each rear latch. A typical design might provide about 11/2 inch of travel, via the flexibility of a coil spring.
The limited degree of travel for the rear mounts provided by such prior arrangements has not been sufficient to accommodate the degree of frame twisting that normally occurs in a tractor truck. Also, these arrangements have failed to accommodate the total motion of the frame with respect to the cab, which is a great deal more than simple up-and-down movement. The motion is actually a combination of side-to-side and rocking movement. The shape of the mounts and their receiving surfaces, also, have not been adequate to accommodate the rocking and side-to-side movement while at the same time serving to center the truck cab rear under normal operating circumstances.
U.S. Pat. No. 3,825,295, assigned to the assignee of the present invention, discloses a cab rear mount and latch mechanism wherein, at each side, a cradle is provided on the frame, with a flat bottom and outwardly extending arms. The cradle receives a complimentary shaped cab mount member which is designed to rest against the two angled arms of the cradle. Co-operating with this mount arrangement, as disclosed in the patent, is a latching assembly which provides for a degree of travel principally in the up and down direction, with automatic releasing of the latches on the initial pressurization of the hydraulic system which lifts the cab on its tilt axis. While the system of this patent has performed satisfactorily, and while the present invention is based in part upon the latching mechanism of the patent, there has been a need for a mount and latch system which permits a greater degree of rocking and side-to-side movement of the cab with respect to the frame (or vice versa). Cradles having two arms tend to restrain the relative motion of the cab and frame, particularly since downward and outboard movement of the cab mount is prohibited. High stresses have often developed in the cab with such systems. Occasionally, one of the arms of each cradle has tended to break off because the relative movement has been overly restrained.
There is a theoretical ideal path for relative motion of the cab body with respect to the frame at the rear of the cab during twisting movement of the frame. Such as ideal path is one which keeps the cab free from stresses during frame twisting. If at each cab mount, the cab could be permitted to follow or approximately follow this ideal path during frame twisting, with a latching mechanism which still securely holds the mounts to the frame, cab stresses could be virtually eliminated without hindering the normal twisting of the frame.
It is therefore among the objects of the invention to provide an improved rear cab mounting and latching system for a tilt cab truck wherein cab stresses are virtually eliminated under normally stressful frame twisting conditions by permitting the cab to follow approximately its ideal free-movement path with respect to the frame. Another important and related object is to provide a new type of cab latch which has the safe and automatic latching and unlatching features of U.S. Pat. No. 3,825,295 but which also provides for a great deal more rocking and side-to-side movement of the cab, with respect to the frame. A further related object is to accomplish these goals without complex and expensive mechanical schemes.